Promotion of oxygen reduction and evolution
by applying a nanoengineered hybrid catalyst
on cobalt free electrodes for solid oxide cells
Abstract
A key requirement for the widespread commercialization of solid oxide cell (SOC) technology is to develop cost-effective oxygen electrodes with sufficiently high electro- catalytic activity and durability at intermediate temperatures (600–750 °C). Here we report a remarkable enhancement of electro-catalytic activity of cobalt-free La0.6Sr0.4FeO3–δ (LSF)
electrode by applying a nanoengineered hybrid catalyst coating composed of nanoparticles of Ce0.85Gd0.15O2-δ (CGO) and PrOx via infiltration. Different from the conventional
infiltration with a precursor of metal nitrate, here a mixture solution of colloidal CGO nanocrystals and Pr(NO3)3 is used for infiltration to enable the desired nanoengineered
architecture. The hybrid-catalyst-coated LSF oxygen electrode displays a very low polarization resistance of 0.017 Ω cm2 at 750 °C, about one order of magnitude lower than
that of the bare LSF (0.197 Ω cm2). Furthermore, the hybrid-catalyst-coated LSF electrode
shows excellent performance in both fuel cell and electrolysis modes, while it also offers good durability over more than 1000 h of operations. The superior performance and durability are attributed to the combined effects of accelerating oxygen surface exchange kinetics by PrOx and enhancing the surface area for catalytic reaction by the nanoporous
architecture of the catalyst coating. This work not only opens the opportunity for applying the cobalt-free oxygen electrode in SOCs but also proposes a smart co-infiltration strategy to develop highly active and robust catalyst coating by combinedly tunning the composition and morphology, which may also be applicable to other devices such as metal-air batteries and membrane reactors.
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5.1 Introduction
Reducing the operating temperature to the intermediate temperature range (600–750 °C) will be highly beneficial to enable the widespread commercialization of the SOC technology. One of the technical challenges for operating SOCs in this temperature range is to ensure sufficiently high electro-catalytic activity toward the oxygen reduction reaction (ORR, when operated in fuel-cell mode) and the oxygen evolution reaction (OER, when operated in electrolysis mode) as well as good durability. During the past decades, substantial efforts have been made, leading to a shift in oxygen electrode materials from the conventional electronic conducting La1-xSrxMnO3 (LSM) compound to mixed ionic-electronic conductors
(MIECs). In particular, many cobalt-containing perovskite-type oxides, such as La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) [23], La0.6Sr0.4CoO3−δ (LSC) [25],
Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) [27], Sm0.5Sr0.5CoO3-δ (SSC) [26], and PrBa0.8Ca0.2Co2O5+δ
(PBCC) [77], have been developed, and have shown to provide favorable catalytic activity at intermediate temperatures. Nevertheless, these cobalt-containing materials are not without problems. The materials show a large thermal expansion mismatch with commonly used electrolyte materials, i.e., yttria-stabilized zirconia (YSZ) and scandium-stabilized zirconia (SSZ); the chemical stability of them is not excellent; the volatilization and diffusion of cobalt during the high-temperature sintering process cause various cell fabrication issues [30-32]. Furthermore, cobalt is price volatile and it may become resource-limited due to the rocketing demand of cobalt oxide for lithium-ion batteries [31]. Finally, it is considered carcinogenic. All these factors push the development of cobalt-free electrodes for SOCs. LaFeO3-based oxides are attractive alternatives because of their similar thermal
expansion coefficients to that of the conventional electrolyte materials and improved chemical stability as well as the high abundance and low cost of iron. [156, 157] However, LaFeO3-based materials exhibit relatively low catalytic activity for ORR and OER in
comparison with cobalt-containing materials. For example, La0.5Sr0.5FeO3-δ electrode was
reported to show a polarization resistance (Rp) of 0.79 Ω cm2 at 700 °C [158], more than 2
times that of the most commonly used LSCF electrode (0.34 Ω cm2) under the same
conditions[159]. The Rp of La0.5Sr0.5FeO3-δ electrode can be reduced to a value comparable
to that of LSCF by substitution with other elements than Co on the Fe site. A value of 0.35 Ω cm2 has been reported for La
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matching that of LSCF is still ~4 times higher than that of more cobalt-rich composite like PBCC electrodes (0.071 Ω cm2) under the same conditions [77], and ~50 % higher than that
of BSCF electrode (0.22 Ω cm2) at a temperature of 650 °C. [160]
Recently, surface modification via infiltration of oxygen electrodes with catalytically- active nanoparticles/coating has emerged as an effective approach to enhance the activity. [77, 78, 80, 81, 161] Several kinds of catalyst coatings have been deposited onto LSCF electrode surfaces via infiltration. In such catalyst-coated LSCF electrodes, the porous LSCF backbones provide a highly continuous pathway for the transport of both oxygen ions and electrons while the catalyst coatings offer enhanced surface oxygen exchange kinetics and tolerance to contaminant poisoning and in some cases improved durability. [78, 161] For instance, a coating of PrSrCoMnO6–δ (PSCM) on LSCF reduced the Rp from 0.126 to 0.093
Ω cm2 at 750 °C. [162] A multi-phase catalyst coating has also been reported to not only
dramatically reduce the Rp from 2.57 Ω cm2 (for bare LSCF) to 0.312 Ω cm2 at 600 °C but
also to enhance the durability. [79] In the above examples, the coatings were introduced by infiltration of nitrate solutions that were subsequently decomposed via heat treatment. By this approach the overall composition of the coating is well controlled but the phase and the morphology of coating, which is also important for performance, is not well controlled. It will among other things depend on the temperature of the firing step.
Here, we report on La0.6Sr0.4FeO3–δ (LSF)-based electrodes that can be strongly improved
to a level allowing operation between 650 and 750 °C by applying a nanoengineered hybrid catalyst coating realized through a nonconventional infiltration route. The hybrid coating is composed of nanoporous layer of nanoparticles of Ce0.85Gd0.15O2-δ (CGO) and PrOx. The
coating is achieved by infiltrating the porous LSF with a solution of colloidal CGO nanocrystals and Pr(NO3)3. PrOx was included because of its good catalytic activity towards
the ORR. [110, 163] The resulting hybrid-catalyst coated LSF electrode displayed a very low Rp of 0.017 Ω cm2 at 750 °C. This performance is comparable to those of the high-
performance cobalt-containing electrodes mentioned previously. The effect of coating on oxygen surface exchange kinetics was investigated by electrical conductivity relaxation (ECR) measurements. Moreover, the electrode demonstrated excellent durability in full cells for both practical fuel-cell and electrolysis operations.
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5.2 Results and discussion
5.2.1 Microstructure of LSF electrodes
CGO nanocrystals were synthesized through continuous hydrothermal flow synthesis (CHFS), as reported previously. [164] The shapes and exposed facets of the CGO nanocrystals were examined using transmission electron microscopy (TEM). A typical TEM image of CGO nanocrystals in Figure 5.1a shows crystallites of well-defined octahedral shapes with sizes of 25-45 nm. Figure 5.1b shows a high-resolution transmission electron microscopy (HRTEM) image of a typical octahedron, indicating that the octahedron was dominated by (111} facets with the lattice fringes at 0.31 nm. [164, 165]
Shown in Figure 5.1c is a typical SEM image of the bare LSF electrode backbone after firing at 1100°C for 5 h, showing smooth surfaces and clear grains. In Figures 5.1b-d, images of CGO, PrOx, and the hybrid-catalyst (CGO+PrOx)-coated LSF electrodes are
displayed. The electrodes were prepared by infiltrating a colloidal CGO nanocrystal solution, Pr(NO3)3 solution, and mixture of the two into the porous LSF backbones and calcinating at
750 °C for 4 h. The loading of coating was 9 wt.% relative to the porous LSF backbones.
Figure 5.1d shows that CGO nanocrystals are uniformly deposited on the LSF grains and
that they have maintained their shapes. Once coated with the PrOx catalyst, the continuous
and dense film is observed on the surface of the LSF grains, making it appear rougher (Figure 5.1e). Figure 5.1f shows that the hybrid catalyst covers the LSF very well with particle size of 25–60 nm and forms a nanoporous coating. Compared to the dense PrOx
coating obtained after Pr(NO3)3 infiltration, this nanoporous hybrid catalyst coating achieved
by co-infiltration of colloidal CGO nanocrystals and Pr(NO3)3 may largely enhance the
surface area for the catalytic reaction. This was demonstrated by the Brunauer-Emmett- Teller (BET) results, in which the measured surface areas for the bare LSF, CGO-coated LSF, PrOx-coated LSF, and hybrid-catalyst-coated LSF electrodes were 1.2, 2.9, 2.1 and 3.0
m2 g−1, respectively. In addition, introducing CGO nanocrystals decreases the use of praseodymium which is costly.
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Figure 5.1: (a) TEM and (b) HRTEM images of CGO nanocrystals. Cross-sectional SEM
images of four types of LSF electrodes. (c) Bare LSF, (d) CGO-coated LSF, (e) PrOx-
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5.2.2 Performance and durability of the LSF electrodes
The electrochemical performance of the bare LSF, CGO-coated LSF, PrOx-coated LSF, and
hybrid-catalyst (CGO and PrOx)-coated LSF electrodes was evaluated by measuring the
electrochemical impedance spectra (EIS) of symmetrical cells with a configuration of electrode/electrolyte/electrode. Figures 5.2a and 2b compare Nyquist plots of EIS data for the four types of LSF electrodes obtained at 750 and 650 °C, respectively. Clearly, all of the coated LSF electrodes show significantly reduced polarization resistances (Rp) when
compared to the bare LSF. The hybrid-catalyst-coated LSF electrode displays the lowest Rp among them. For instance, the Rp value of the hybrid-catalyst-coated LSF at
750 °C is 0.017 Ω cm2, about one order of magnitude lower than that of the bare LSF (0.197
Ω cm2), ∼81% lower than that of CGO-coated LSF (0.089 Ω cm2), and ∼39% lower than
that of PrOx-coated LSF (0.028 Ω cm2) under the same conditions.
The performance of the hybrid-catalyst-coated LSF electrode reported here is comparable to those of the high-performance cobalt-containing electrodes reported recently, including LSCF or LSCF nanofiber [159], BSCF-Ce0.9Gd0.1O2−δ (CGO10) mixture or nanofibers [160],
SrTi0.3Fe0.63Co0.07O3−δ (STFC) [96] and PBCC [77] as well as several representative surface-
coated-electrodes such as PSCM-coated LSCF [162], PrNi0.5Mn0.5O3 (PNM) thin film and
PrOx nanoparticles -coated LSCF [78], multi-phase catalysts (composed of BaCoO3−x (BCO)
and PrCoO3−x (PCO) nanoparticles and a conformal PBCC thin film)-coated LSCF [79], and
SSC-coated LSCF/CGO10 [81] (Figure 5.2c). In one exception, Nicollet et al. reported even better performing cobalt-free electrodes, i.e., Pr6O11-coated CGO10 electrodes, which
achieved polarization resistances as low as 0.028 Ω cm2 at 600 °C, prepared by infiltrating
Pr(NO3)3 into the porous CGO10 backbone. [110] Compared to the hybrid-catalyst-coated
LSF electrode developed in this work, the above Pr6O11-coated CGO10 electrodes are with
a higher loading of Pr6O11 catalysts (30 wt.% relative to the CGO10 backbone) and
calcinated at a lower temperature of 600 °C for 2 h before being tested. Note that the calcination temperature and catalyst loading can significantly influence the performance of infiltrated electrodes. [74, 166] The performance of the hybrid-catalyst-coated LSF electrode may be further improved by, for example, using CGO nanocrystals with smaller size or optimizing the component ratio or loading of the hybrid catalyst. Such studies will be part of our future work.
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Figure 5.2: Electrochemical performance and durability of symmetrical cells with four types
of LSF electrodes measured under OCV conditions. Nyquist plots of EIS data measured at (a) 750 °C and (b) 650 °C. (c) Comparison of Rp of hybrid-catalyst-coated LSF electrode
with some high-performance cobalt-containing electrodes reported recently. (d) DRT plots of EIS data for four types of LSF electrodes measured at 750 °C. (e) Durability on
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symmetrical cells with four types of LSF electrodes at 700 °C under OCV condition. (f) DRT plots at 0 h and 120 h.
The kinetics of the electrochemical processes on these LSF electrodes were investigated by examining the distribution of relaxation time (DRT) of the EIS data. DRT analysis is a useful tool for deconvoluting the complex EIS data to separate the contributions of some key steps from the total Rp of electrode reactions. In general, each peak in DRT plots represents
an electrode process, and its integral area corresponds to the resistance contribution of that process. The higher the characteristic frequency of the peak, the faster the relaxation of the corresponding process. Shown in Figure 5.2d is the comparison of DRT plots for symmetrical cells with the four types of LSF electrodes. Figure 5.S1 contains Nyquist and DRT plots of bare LSF electrode under different oxygen partial pressures (pO2) and of the
hybrid-catalyst-coated electrode at different temperatures. In these DRT plots three distinct peaks are identified in the frequency range of 0.3–100 kHz, they are denoted as low frequency (LF), intermediate frequency (IF), and high frequency (HF). The LF peak has little dependence on temperature while the other peaks are characterized by a pronounced thermal activation (Figure 5.S1b). This strongly indicates that the LF peak is related to gas diffusion within the pores of the electrodes since gas diffusion process is only mildly temperature dependent. [79] The IF peak has a strong dependence on temperature and pO2 (Figures 5.S1b
and S1d), indicating that the IF peak is associated with the oxygen surface exchange processes including oxygen adsorption/desorption, dissociation, and/or surface diffusion. [77, 79]The insensitivity of the HF peak to pO2 (Figure 5.S1d)suggests that the HF peak is
likely related to transport of O2- within the LSF backbone and/or across the electrode/electrolyte interface, which are pO2 less dependent processes. [77, 79] It is clearly
seen in Figure 5.2d that at 750 °C the resistance of IF peak (surface exchange processes) dominates the total Rp of the bare LSF electrode, and that this peak decreases significantly
by introducing coatings on the electrode. Also, its characteristic frequency shifts to higher values. The hybrid-catalyst-coated electrode has the highest characteristic frequency and the lowest resistance of the IF peak.
These results are consistent with results of characterizations of the surface exchange coefficient (k) from electrical conductivity relaxation (ECR) measurements. Results of ECR experiments on as prepared LSF bars and surface coated ones are reproduced in Figure 5.S2. The k value of a bar of LSF is significantly increased after coating with catalysts, and the
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hybrid catalyst coated LSF possesses the highest k value. For example, the k values at 750 °C are 2.07 × 10-4, 9.71 × 10−4, 1.67 × 10−3 cm s−1 for the bare LSF, PrOx-coated LSF, and the
hybrid-catalyst-coated LSF, respectively. The activation energy (Ea) for oxygen surface
exchange decreases from 2.49 eV on bare LSF to 0.68 eV on PrOx-coated LSF, and 0.96 eV
on hybrid-catalyst-coated LSF. The lowest Ea of the PrOx-coated LSF electrode may suggest
that PrOx has the highest intrinsic activity for oxygen exchange, while the hybrid-catalyst-
coated LSF electrode shows the highest k in ECR and lowest Rp in the symmetrical cell due
most likely to its unique nanostructure, which combines the effects of high intrinsic activity of PrOx and the large surface area enabled by the CGO nanocrystals.
The short-term durability of four types of LSF electrodes was investigated using symmetrical cells aged at 700 °C. The evolution in Rp with time is summarized in Figure
5.2e. The order in degradation rate is: bare LSF (0.49 mΩ cm2 h-1) > PrOx-LSF (0.47 mΩ cm2 h-1) > CGO-LSF (0.23 mΩ cm2 h-1) > hybrid-LSF (0.14 mΩ cm2 h-1).Figure 5.2f shows
a comparison of DRT plots at 0 h and 120 h for the four types of symmetrical cells. After 120 h of tests, all of the DRT plots display a shift in characteristic frequency to lower frequency on the IF peak and an accompanying increase of the corresponding integral resistance, reflecting a reduced rate of the surface exchange process on the electrodes. Remarkably, the CGO-coated LSF and hybrid-catalyst-coated LSF electrodes show a significantly smaller increase in the IF peak resistance in comparison with that of PrOx-
coated LSF, revealing a positive effect on electrode durability of introducing CGO nanocrystals into the coating.
5.2.3 Performance and durability of full cells incorporating the
hybrid-catalyst-coated LSF oxygen electrodes
To evaluate the performance of the hybrid-catalyst-coated LSF oxygen electrode in an actual full cell, fuel-electrode-supported cells with a configuration of Ni/YSZ support | Ni/YSZ fuel electrode | YSZ electrolyte | CGO10 barrier | hybrid-catalyst-coated LSF oxygen electrode (hybrid-catalyst-coated LSF cell) were fabricated and tested under both fuel cell and electrolysis operating conditions. Cross-sectional views of the cell components are shown in Figure 5.S3. Figure 5.3a shows the typical I-V-P curves of the cell tested under
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representative fuel cell conditions with 4%H2O-96%H2 fed to the fuel electrode and dry air
to the oxygen electrode. For comparison, the performance of a cell with a bare LSF electrode (LSF cell) was also characterized under identical test conditions. The hybrid-catalyst-coated LSF cell delivers peak power densities (PPDs) of 1.57, 1.15, 0.67 W cm−2 at 750, 700, and 650 °C, respectively, about 70, 130, and 170 % higher than those of the LSF cell (0.94, 0.50, 0.25 W cm−2 at 750, 700, and 650 °C, respectively).
Figure 5.3: Electrochemical performance on Ni/YSZ fuel-electrode-supported cells with the
bare LSF and hybrid modified LSF oxygen electrodes. (a) Voltage and power density versus current density (I-V-P) measured at 650–750 °C with 4%H2O-96%H2 fed to the fuel
electrode and dry air to the oxygen electrode (fuel cell mode). (b) Voltage versus current density (I-V) measured at 650–750 °C with 50%H2O-50%H2 fed to the fuel electrode and
dry air to the oxygen electrode (electrolysis mode). (c) Nyquist plots of EIS data recorded at 750 °C under OCV. (d) Comparison of Rohm and Rp under OCV.
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Figure 5.3b shows the I-V curves of the cells tested under electrolysis conditions with 50%H2O-50%H2 fed to the fuel electrode and dry air to the oxygen electrode. At the
preferred electrolysis operation voltage of 1.3 V, the current densities of the hybrid-catalyst- coated LSF cell are 1.62, 1.01, 0.54 A cm−2 at 750, 700, and 650 °C, respectively; these values are about 90, 130, and 170 % higher than those of the LSF cell (0.86, 0.44, 0.20 A cm−2 at 750, 700, and 650 °C, respectively). In light of the results on symmetrical cells, this significantly improved performance observed on the hybrid-catalyst-coated LSF cell under realistic fuel cell and electrolysis operating conditions is as expected. The results demonstrate that surface modification by the suggested nano-engineered hybrid catalyst coating can be effectively transferred to real cell architecture resulting in very well performing cells. Encouragingly, the behavior of the hybrid-catalyst-coated LSF cell reported here competes well with those of high-performance SOCs based on cobalt- containing electrodes [77, 78, 81, 162, 167].
Figure 5.3c shows typical EIS data of the hybrid-catalyst-coated LSF cell and the LSF cell acquired at 750 °C under OCV conditions. The polarization resistance (Rp) is
significantly reduced by introducing the hybrid-catalyst coating, meanwhile the ohmic resistance (Rohm) also decreases slightly. For example, the Rp of the hybrid-catalyst-coated
LSF cell is 0.098 Ω cm2 under 50%H
2O-50%H2 fed to the fuel electrode, as only 23 % of
that of the LSF cell (0.429 Ω cm2) under identical test conditions. Moreover, the hybrid-
catalyst-coated LSF cell shows lower activation energy of Rp compared with the LSF cell
(Figure 5.3d), consistent with the more significant improvement in performance at low temperature observed from the I-V curves in Figures 5.3a and 3b.